Design rules for advanced generation microlithography (i.e., beyond 193 nm immersion lithography and into next generation optics such as e-beam, X-ray, and extreme ultraviolet (EUV) lithography operating at a very short wavelength of 13.4 nm) are trending toward smaller and smaller dimensions, for example, 30 nm and below. In general, depth of focus (DOF) necessarily decreases with higher resolution due to the higher numerical aperture (NA) and therefore resist thickness also decreases commensurate to the smaller and smaller feature sizes. With narrower linewidths and thinner resist films, consistency issues such as line width roughness (LWR) and resolution take on increasing significance limiting the performance and usefulness of photoresists. These phenomena are of interest in the fabrication of semiconductor devices; for example, excessive LWR can lead to poor etch and lack of linewidth control in, for example, transistor and gate architecture, potentially causing short circuits and signal delay.
Achieving such feature may be improved with polymers having well-controlled properties of composition, molecular weight, and polydispersity. Acrylate-based EUV photoresist polymers may be synthesized by a modified free radical polymerization techniques in which careful control of monomer and initiator feed rates helps to control the composition, but termination and chain transfer reactions may cause composition drift throughout the polymerization and a relatively broad distribution of molecular weights. Because such diversity is a significant factor affecting resist solubility, a broad composition and molecular weight distribution are undesired.
Controlled radical polymerization techniques have become more practical in the last decade, and acrylate polymers with polydispersities as low as 1.05 may be prepared. This invention relates to the modification of polymerization conditions for EUV photoresists to employ a dithioester chain transfer agent (CTA), which successfully controls molecular weight distributions. As shown in the Example section, polydispersities as low as 1.2-1.3 can be achieved compared to PDIs of approximately 2.0 for traditional free radical polymerizations. Additional benefits have been realized using controlled radical polymerization, such as the ability to accurately target a specific molecular weight based on the amount of CTA and the option to end-functionalize polymers due to the living-like behavior of the polymerization. Furthermore, controlled radical polymerizations can be run to high conversion (>80%) without the deleterious effects typically observed for free radical polymerizations at high conversion. This allows for improved polymer yields and easier purification of polymer from unreacted monomer.
As described in Proc. of SPIE Vol. 6923, 2008, 69232E-1-69232E-9, reversible addition fragmentation transfer (RAFT) polymerization techniques have been used to produce photoresist polymers for 193 nm lithography. However, photoresist polymers having improved (i.e., decreased) linewidth roughness are still desirable for EUV applications.